Relief printing plates, such as flexographic plates, falls into two main categories: (1) those that are processed using aqueous solutions to remove unexposed photopolymer, and (2) those that need to be processed using some other chemical solvent. In recent years, flexographic printing plates using an aqueous-processable photopolymer are gaining more market interest because of their environmentally-friendly characteristics. They have the additional advantage that they can reduce organic solvent exposure at the workplace. Aqueous-processable printing plates are sometimes referred to as aqueous-washable printing plates because the processing typically involves washing off the unexposed photopolymer.
There are also two main types of aqueous-processable flexographic printing plates: (1) those that are processable by dissolution of the photopolymer using a strong alkaline solution (i.e., having a pH>11), and (2) those that are processable by dispersion of the photopolymer using a processing solution including a dispersing agent (typically having a pH<11).
Aqueous-processable flexographic printing plates may be processed (i.e., “washed”) by a number of methods. For example, U.S. Pat. No. 5,124,736 (Yamamoto et al.), entitled “Process and apparatus for developing photopolymer plate,” describe systems which form the relief by spraying processing solution (i.e., “washout solution”) under pressure onto the printing plate, and systems which form the relief by rubbing a brush against the printing plate in the presence of the processing solution, thereby dissolving the unexposed portions in the processing solution. Yamamoto et al., describe a system in which processing solution is filtered and recirculated to the plate processor after a full batch of platemaking.
As noted by U.S. Pat. No. 6,247,856 (Shibano et al.), entitled “Developing system of photosensitive resin plates and apparatus used therein,” photopolymer (i.e., resin) can build up in the used processing solution after processing a number of printing plates. This can cause various problems, such as decreasing the speed of development, and the dispersed resin forming scum which adheres to the plates and the brush. This can require frequent disposal of the used processing solution and preparation of a fresh processing solution. Shibano et al. discloses the addition of fresh processing solution to a processing unit, while removing part of the resin-containing processing solution to keep the resin content of the processing solution substantially constant.
In order to remove debris that becomes attached to the surface of the printing plate, a rinsing station can be employed after the main plate processing step. U.S. Patent Application Publication No. 2009/0013888 (Danon), entitled “Methods and means relating to photopolymer printing plates,” discloses processing a printing plate using a processing solution, followed by rinsing with water in a rinsing station. Used processing solution is recycled back to the processor after filtration. Waste water from the rinsing station may also be recycled back to the processor after filtration.
U.S. Pat. No. 5,828,923 (Harabin et al.), entitled “Apparatus and method for processing water wash photopolymer solution,” disclose directing used processing solution into a holding tank, and adding a coagulant to coagulate the solid content for disposal.
European Patent 0586470B1 (Danon), entitled “Preparation of photopolymerised elastomeric printing plates” disclose a processing system including (a) a wash-out section where unexposed areas of the plate are removed; (b) a rinse section; (c) an excess water-removing section; (d) a light-finishing section where the plate is exposed by UV light to reduce the stickiness of the plate surface; and (e) a drying section.
European patent 0586483B1 (Danon), entitled “Method and Apparatus for washing-out printing plates,” discloses a system for processing printing plates where processing solution is directed through a spray bar along downwardly directed bristles of a washout brush.
It has been found that even when utilizing a plate processor that includes a rinse operation, such as that disclosed in the aforementioned U.S. Patent Application Publication No. 2009/0013888, there can still be significant problems with plate defects after the production of only a few plates. The occurrence of plate defects is particularly problematic with the increasingly popular photopolymer plates with include micro-texture on the raised plate surface (i.e., the printing surface). As discussed in U.S. Pat. No. 8,399,177 (Stolt et al.), entitled “Enhanced printing plate,” the micro-texture is beneficial to enhance print density and uniformity. In such cases, the debris particles tend to accumulate on the micro-texture surface of the printing plate, which results in unacceptable print defects after processing a small number of printing plates.
There remains a need for an improved aqueous processing method for flexographic printing plates that enables processing a larger number of defect-free printing plates before it is necessary to replace the used processing solution.